In recent years a parking assist device has been studied and developed. For assisting parking operation, this device uses a camera mounted on a vehicle to observe the surroundings of the vehicle, detects the position of a parking frame marked by white or yellow lines drawn on a road surface from an observed image, and calculates a relationship between the position of the vehicle and the line markings according to the detected position of the parking frame.
Such a system makes use of the feature of the parking frame having brightness largely different from that of the road surface, for example, to detect the position of a parking frame by detecting pixels having brightness largely different from that of the road surface and applying straight lines to the columns of detected pixels (Patent Literature 1, for instance).